307013-003 Datasheet, PDF (486/848 Pages) Intel Corporation – Intel I/O Controller Hub 7

UHCI Controllers Registers
11.2.6
SOFMOD—Start of Frame Modify Register
I/O Offset:
Default Value:
Base + (0Ch)
40h
Attribute:
Size:
R/W
8 bits
This 1-byte register is used to modify the value used in the generation of SOF timing on
the USB. Only the 7 least significant bits are used. When a new value is written into
these 7 bits, the SOF timing of the next frame will be adjusted. This feature can be
used to adjust out any offset from the clock source that generates the clock that drives
the SOF counter. This register can also be used to maintain real time synchronization
with the rest of the system so that all devices have the same sense of real time. Using
this register, the frame length can be adjusted across the full range required by the
USB specification. Its initial programmed value is system dependent based on the
accuracy of hardware USB clock and is initialized by system BIOS. It may be
reprogrammed by USB system software at any time. Its value will take effect from the
beginning of the next frame. This register is reset upon a host controller reset or global
reset. Software must maintain a copy of its value for reprogramming if necessary.
Bit
Description
7 Reserved
SOF Timing Value — R/W. Guidelines for the modification of frame time are contained
in Chapter 7 of the USB Specification. The SOF cycle time (number of SOF counter clock
periods to generate a SOF frame length) is equal to 11936 + value in this field. The
default value is decimal 64 which gives a SOF cycle time of 12000. For a 12 MHz SOF
counter clock input, this produces a 1 ms Frame period. The following table indicates
what SOF Timing Value to program into this field for a certain frame period.
Frame Length (# 12 MHz
Clocks) (decimal)
SOF Timing Value (this register)
(decimal)
11936
0
6:0
11937
1
—
—
11999
63
12000
64
12001
65
—
—
12062
126
12063
127
486
Intel ^® ICH7 Family Datasheet